1. Field of the Invention
This disclosure relates to a semiconductor memory device, and more particularly, to an on-chip temperature detector linearly detecting a sensed temperature, a temperature detecting method thereof, and a refresh control method using the same.
2. Description of the Related Art
In general, a semiconductor device has operating characteristics that depend on temperature. As shown in FIG. 1, typical operating characteristics of the semiconductor device include a supply current IDD and an access time tACCESS. The access time increases (A) as temperature increases, and the supply current IDD increases (B) as temperature decreases.
Temperature dependent characteristics such as these are important for volatile memory devices such as DRAMs. Leakage currents in DRAMs increases as temperature increases. This deteriorates a data sustain characteristic, reducing a data sustain time tST. Accordingly, as temperature increases the DRAM requires more frequent refresh operations.
The development of electronic technologies has enabled the design and cost-effective manufacture of portable electronic devices. Such portable electronic devices include pagers, cellular phones, music players, calculators, lap-top computers, PDAs, and so on. The portable electronic devices generally require DC power, and thus, one or more batteries are used as an energy source to supply the DC power to the portable electronic devices.
In a battery-operated system, it is important to reduce the power consumption. To achieve this, circuit components included in the system are turned off during a sleep mode used for power saving. However, a DRAM included in the system should continuously refresh data stored in DRAM cells in order to preserve the data.
One of the attempts to reduce power consumed in the DRAM is to vary a refresh period with temperature. In FIG. 1, when the refresh period is increased to reduce a refresh clock frequency in a low temperature region where consumption current is increased power consumption is decreased. Accordingly, a temperature detector for detecting the internal temperature of the DRAM is required.
FIG. 2 is a circuit diagram of a conventional temperature detector 200. Referring to FIG. 2, the temperature detector 200 includes a proportional to absolute temperature current generator (referred to as “PTAT current generator” hereinafter) 210, a complementary to absolute temperature current generator (referred to as “CTAT current generator” hereinafter) 220, and a comparator 230.
The PTAT current generator 210 includes first and second PMOS transistors MP1 and MP2, first and second NMOS transistors MN1 and MN2, a resistor R, and first and second diodes D1 and D2. The first and second PMOS transistors MP1 and MP2 have the same size and form a first current mirror. The first and second NMOS transistors MN1 and MN2 have the same size and form a second current mirror. The sizes of the first and second diodes D1 and D2 have the ratio of 1:M.
Since the input and the output of the first current mirror formed by the first and second PMOS transistors MP1 and MP2 and the output and the input of the second current mirror formed by the first and second NMOS transistors MN1 and MN2 are respectively connected to each other, a current Ia1 and a current Ia2 are identical to each other. The ratio of Ia1 to Ia2 is 1:1.
In general, a turn-on current ID of a diode is as follows.ID=Is*(eVD/VT−1)≈Is*(eVD/VT)  [Equation 1]
Is represents reverse saturation current of the diode, VD is a diode voltage, and VT is a temperature voltage represented by kT/q. Where T is the temperature, k is a constant and q is the change of an electron. Accordingly, the current Ia1 flowing through the first diode D1 is as follows:Ia1=Is*(eVD1/VT)  [Equation 2]The first diode voltage VD1 is as follows:VD1=VT*ln(Ia1/Is)  [Equation 3]The second diode voltage VD2 is as follows:VD2=VT*ln(Ia2/(Is*M))  [Equation 4]
Since the current Ia1 and the current Ia2 are identical to each other, the first diode voltage VD1 and current temperature voltage NOC0 becomes almost identical to each other. Accordingly, the following equation is obtained:V(NOC0)=VD1=VD2+Ia2*R  [Equation 5]
When VD1 and VD2 of Equation 5 are replaced with Equations 3 and 4, respectively, the following equation is obtained:VT *ln(Ia1/Is)=VT*ln(Ia2/(Is*M))+Ia2*R  [Equation 6]
Accordingly, the current Ia1 is as follows.Ia2=VT*ln(M)/R  [Equation 7]
Thus, the current Ia1 is proportional to temperature. That is, the PTAT current generator 210 generates the current Ia1 proportional to the temperature of the PTAT current generator 210.
The CTAT current generator 220 includes a third PMOS transistor MP3, a third NMOS transistor MN3, a plurality of resistors Raa, RU1 through RU5, and RD1 through RD5, and a plurality of switching transistors TU1 through TU5 and TD1 through TD5.
The switching transistors TU1 through TU5 and TD1 through TD5 are selectively turned on/off in response to trip temperature control signals AU1 through AU5 and AD1 through AD5.
The resistors RU1 through RU5 and RD1 through RD5 are respectively connected to the switching transistors TU1 through TU5 and TD1 through TD5 Accordingly, any switching transistors TU1 through TU5 and TD1 through TD5 that are turned on short circuit the respective resistors PU1 through RU5 and RD1 through RD5.
If the current Ia1 and current Ia2 are almost identical to one another, a VA node voltage and VB node voltage of the PTAT current generator 210, and a VC node voltage of the CTAT current generator 220 become almost identical to one another. In Equations 3 and 4, the voltage VT is increased as temperature is increased. However, the current Is is also increased. As a result, the diode voltage is reduced as temperature is increased and hence the node voltages VA and VC are decreased. Accordingly, the current Ib flowing through the resistors Raa, RU1 through RU5 and RD1 through RD5 is decreased as temperature is increased. Thus, the CTAT current generator 220 generates a current that varies inversely proportional to temperature.
The comparator 230 compares the current temperature voltage NOC0 to a sensed temperature voltage NOC1. The current temperature voltage NOC0 and sensed temperature voltage NOC1 are determined by the current Ia1 and current Ib1, respectively. When the point at which the current Ia1 becomes identical to the current Ib1 is found, as shown in FIG. 3, based on the current temperature voltage NOC0 and sensed temperature voltage NOC1, the temperature detector 200 detects the current temperature.
Referring to FIG. 3, consider a target temperature of the temperature detector 200 of 45° C. When the current Ib is smaller than the current Ia1, the trip temperature control signals AU1 through AU5 and AD1 through AD5 of the CTAT current generator 220 are selectively enabled to decrease the resistance value of the CTAT current generator 220 such that the current Ib is increased, as shown by direction (C), to make the current Ia1 identical to the current Ib. In contrast, when the current Ib is larger than the current Ia1, the trip temperature control signals AU1 through AU5 and AD1 through AD5 of the CTAT current generator 220 are selectively disabled to increase the resistance value of the CTAT current generator 220 such that the current Ib is decreased, as shown by direction (D), to make the current Ia1 identical to the current Ib.
When the current Ia1 becomes identical to the current Ib at the target temperature, 45° C., the comparator 230 outputs a signal having alternating logic levels of high-low-high-low. Accordingly, the temperature detector 200 detects the current temperature, 45° C.
The temperature detector 200 controls the trip temperature control signals AU1 through AU5 and AD1 through AD5 to adjust the resistance value of the resistor branch of the CTAT current generator 220 to change the sensed temperature, that is, the current Ib. When the resistance value is controlled, a gradient of the sensed temperature is not uniform due to the variation of the resistance value. Accordingly, the gradient of the sensed temperature is non-linear. Furthermore, the temperature detector 200 detects the current temperature of a chip based on a single target temperature, and thus the target temperature is fixed to one value.